This invention generally relates to trip mechanisms for rotary machines, and more specifically to a Belleville spring type of trip mechanism. Even more specifically, the present invention relates to a Belleville spring type of trip mechanism particularly well adapted for use with rotatable shafts which rotate at speeds less than approximately 7000 rotations per minute.
Trip mechanisms are frequently employed with rotary machines to reduce the speed thereof or to completely terminate operation of the rotary machine upon the occurrence of an undesirable operating condition such as excessive shaft speed. One type of trip mechanism which is commonly used is the Belleville spring type, which generally includes a Belleville spring, a pair of mounting collars, and a plurality of weights. A thorough explanation of this type of trip mechanism may be found in U.S. Pat. Nos. 2,973,771 and 3,374,680.
Briefly describing the Belleville spring type of trip mechanism, the Belleville spring is shaped like a dished washer and encircles a rotatable shaft, and the collars of the trip mechanism are connected to the shaft at opposite sides of the Belleville spring to mount the spring on the shaft. Rotary motion is transmitted from the shaft to the spring, for example via frictional contact between the spring and the collars. The weights are secured to the Belleville spring and, in a manner well known to those skilled in the art, the weights and the spring are so designed that the spring will snap overcenter, that is turn inside out, when the shaft speed exceeds a predetermined value. This snapping action is employed to actuate a valve or other control element to stop the shaft or at least reduce the speed thereof. Once shaft speed is reduced to a second predetermined value, the Belleville spring snaps back overcenter toward its unstressed position.
In many applications, Belleville spring type trip mechanisms are relatively simple and inexpensive yet reliable and effective, and this type of trip mechanism has found wide industrial acceptance. Certain design difficulties, however, have been encountered when using Belleville spring type trip mechanisms with slow or medium speed rotary machines, for example machines having a shaft which rotates at less than approximately 7000 revolutions per minute. These difficulties are especially significant if the rotary machine is relatively small.
As alluded to above, the dimensions of the spring, specifically its thickness and diameter, are chosen so that the spring snaps overcenter at a preselected shaft speed. Generally, the larger the ratio of spring diameter to spring thickness, the lower the shaft speed at which the spring will snap overcenter. Thus, to decrease the shaft speed at which the Belleville spring will snap overcenter, the spring diameter may be increased and/or the spring thickness may be decreased. It is commonly preferred, though, that the spring thickness be no less than approximately 1.4 millimeters (0.055 inches) because if the spring thickness is less, the snapping action of the spring may become relatively slow and gradual. With a gradual turning over, the Belleville spring may actuate the control element, which stops or slows down the rotary machine, before or after the shaft speed reaches the preselected spring snapping value. Thus, the accuracy of the trip mechanism is reduced.
Obviously, this difficulty can be overcome by maintaining the spring at least 1.4 millimeters (0.055 inches) thick. Once a Belleville spring has been reduced to this thickness, to decrease further the shaft speed at which the spring will snap overcenter, the spring diameter may be increased. However, there also are limitations to the extent to which the spring diameter may be increased. Specifically, as may be appreciated, besides a shaft overspeed trip mechanism, rotary machines include a multitude of other components such as bearings, housings, and seals. These other components may strictly limit the size of a Belleville spring which may be used with a particular rotary machine. Even if the size of the spring is not strictly limited, using a Belleville spring having a diameter greater than a certain size may require special, costly modifications to the other components of the machine. These size constraints are particularly severe when designing a Belleville spring type trip mechanism for a low or medium speed, small rotary machine. The various components of the rotary machine are comparatively small and closely fitted together, and the preferred spring diameter to spring thickness ratio is relatively large.